I'm always wary of claims of "reverse discrimination" but I honestly can't think of a reason that, all else being equal (important caveat), that a PhD program would pick an American over a foreign student. Seems like the foreigner would be more motivated, more likely to do whatever the advisor asks with no complaints, less likely to complain about the poor working conditions and pay of a grad student, and much less likely to say "screw this" and go to work for anybody.com (an option not available for the non-green-card-holding foreigner).

Speaking english is a certainly a plus, but it seems outweighed by all the other benefits to the advisors that a foreign student brings.

(Full disclosure: I'm an American who's going to apply next year to PhD programs.)

Here's one reason: a lot of state schools (Berkeley, for example) charge a lower tuition rate for in-state residents. Americans can gain state residency after a year or so of living there in a PhD program, but international students are always nonresidents. So supporting an international student is significantly more expensive than supporting a domestic student from the perspective of whatever research grant or departmental fund is paying your tuition.

Seems like the foreigner would be more motivated, more likely to do whatever the advisor asks with no complaints, less likely to complain about the poor working conditions and pay of a grad student, and much less likely to say "screw this" and go to work for anybody.com (an option not available for the non-green-card-holding foreigner).

But, more likely to be pressured by their family into a "prestigious" career that they aren't really interested in, more likely to have the additional pressure of supporting a wife and be thinking about kids, more likely to be unable to learn how to write research papers well in English (this is surprisingly hard)...

Also, I've definitely seen people from abroad use American grad school as a temporary stepping stone to get a job at a big company (like Microsoft) that will deal with the visa stuff. Not saying they were planning it all along, but it happens.

Of all the above, I think the "being able to write polished, professional, technical research papers" is by far the biggest risk factor that neutralizes the benefits of foreign students that you mentioned.